1. Field of the Invention
The present invention relates to a control system and, more particularly, to a control system which enables a reduction in electric power consumption of backup power by making use of data communications between control units.
2. Description of the Relevant Art
In recent years, a lot of ECUs (Electronic Control Units) have been mounted on vehicles for electronic control, and various types of control have been performed. Among these ECUs, an ECU for EFI for performing fuel injection control of an engine, an ECU for ABS for performing drive control of solenoid valves for hydraulic control and the like, an ECU for a transmission for performing drive control of solenoid valves for shifting and the like, an ECU for air bags for controlling spreading of air bag systems, and an ECU for body work for controlling keyless entry and the like are exemplified. The specifications of these ECUs mounted on vehicles very with car models and grades.
Recently, in order to increase the efficiency of control processing between these ECUs and reduce component costs or the like, multiple ECUs have been mutually communicably connected through a communication line so that they can share data of each ECU.
FIG. 11 is a block diagram schematically showing the construction of a control system including conventional ECUs. Reference numeral 60 in the figure represents a control system, which is constituted of ECUs 50a-50f mutually connected through a communication line 12. A vehicular LAN system is constructed of these ECUs 50a-50f. 
The ECU 50a comprises an input circuit 2 to perform input processing of signals from a sensor, a switch and the like, a microcomputer 53 to perform various types of computing based on the input signals captured through the input circuit 2, an output circuit 4 to output control signals computed in the microcomputer 53 to an actuator and the like, a power circuit 5 to provide stable power supply voltage to the microcomputer 53, and a communication circuit 6 for data communications through the communication line 12 with the other ECUs 50b-50f. 
The microcomputer 53 comprises a CPU 53a, a ROM 53b and a RAM 53c. In accordance with programs previously stored in the ROM 53b, the CPU 53a performs various types of computations based on the input signals captured from the input circuit 2 or data read from the RAM 53c, and stores the computation results in the RAM 53c or outputs them through the output circuit 4 to the actuator.
A battery 8 is connected through a power line 9a and an ignition switch 7 to the power circuit 5. The 12V power from the battery 8 is converted to 5V power in the power circuit 5, and the 5V power is provided to a power terminal 9c of the microcomputer 53 by turning on the ignition switch 7. In addition, the battery 8 is connected through a backup power line 9b without the ignition switch 7 involved to the power circuit 5. Even while the ignition switch 7 is in the off position, a prescribed constant voltage to which power is converted in the power circuit 5 is supplied to the power terminal 9c of the microcomputer 53. The hardware of any of the other ECUs 50b-50f except the ECU 50a, is almost the same as the above.
In the control system 60 having the above construction, when the ignition switch 7 is turned off, each microcomputer 53 in the ECUs 50a-50f is provided with electric power through the backup power line 9b and the power circuit 5 from the battery 8 even during the off state of the ignition switch 7. As a result, even while the ignition switch 7 is in the off position, data stored in the RAM 53c of each microcomputer 53 is not destroyed, so that the ECUs 50a-50f each can maintain required data.
In recent years, the number of ECUs mounted on each vehicle has been larger and larger. Although the electric current consumption for maintaining data stored in RAM is small on a single ECU basis, with an increasing number of ECUs mounted on each vehicle, the electric current consumption for maintaining data per vehicle becomes considerably large.
According to the above conventional method, with an increasing number of ECUs mounted, the electric current consumption, or dark current for maintaining memory contents increases, so that there is a high possibility of an early dead battery being caused in cases where a vehicle is transported or let stand with the battery connected.
Although efforts to decrease the dark current for maintaining memory contents are being made on a single ECU basis, there is a limit to these efforts as long as the basic construction of an ECU is not greatly changed.
There is a method wherein an EEPROM, being a nonvolatile memory which can maintain data even after power supply cutoff, is mounted and part of the data to be stored in RAM is stored in the EEPROM. However, in general, since the capacity of an EEPROM is small, the amount of data which can be written therein is limited. When the capacity thereof is made large, it becomes expensive, resulting in the limited uses thereof.